The structures of the conventional LDD transistors are illustrated in FIGS. 1 and 2. The LDD transistor of FIG. 1 is manufactured in the manner described below. That is, a gate oxide layer 3 is grown on a p type substrate 1, and then, a gate 4 is formed by depositing polysilicon. Then a p type pocket 6 and an n- region 7 are formed by ion-implanting boron and phosphorous respectively, and then, a side wall spacer 11 is formed by performing a reactive ion etching after depositing a low temperature oxide layer. Then arsenic (As) is ion-implanted to form an n+ region 8, thereby forming an LDD transistor.
Meanwhile, the LDD transistor of FIG. 2 is manufactured in the manner described below. That is, boron ions are implanted using a mask only into the portion of a channel 12 of a p type substrate 1, and then, a gate oxide layer 3 is grown on the substrate 1. Then a gate 4 is formed by despositing polysilicon, and then, an n- region 6 is formed by ion-implanting phosphorus. Then a side wall spacer 11 is formed by carrying out a reactive ion etching after depositing a low temperature oxide layer, and then, arsenic (As) is ion-implanted to form an n+ region 8, thereby forming an LDD transistor.
In the LDD transistors manufactured in the manners described above, while the sizes of the transistors are reduced, the problem of the punch-through becomes serious. In an attempt to overcome this problem, a p type pocketing is provided or the concentration of the channel 12 is increased on the source and drain regions 7,8. However, in the case where p type pocketings are provided to the source and drain regions, there arises the problem that the junction capacitance between the n+ region and the p type substrate is increased due to the existance of the p type pocket 6.
Meanwhile, in the case where the concentration of the channel is increased, there is a limit in the threshold voltage Vt, and therefore, there is a limit in increasing the channel concentration. Further, even if the threshold voltage Vt is maintained at a proper level, there is the problem that the body effect (the variation of Vt due to the back bias) is increased.